Some small internal combustion engines for handheld power tools such as chain saws, grass trimmers, weed trimmers, leaf blowers, and the like have carburetors with an internal accelerator pump which supplies additional fuel to the operating engine in certain operating conditions. The accelerator device can temporarily increase the amount of fuel delivered to the engine when the throttle valve is opened fully for improving the acceleration of the engine. This additional fuel is needed to smoothly and rapidly accelerate the engine without stumbling, particularly when the engine is under a load.